In a few problems of advanced mechanics the integrals. The moment of inertia of a composite body is the sum of the moments of inertia of the parts of the body, the same axis of reference being used for each part. If the entire area were concentrated at a point whose distance from the axis or pole is equal to the radius of gyration k, the moment of inertia of the concentrated area would be equal to that of the original area. I Areal moments of inertia. Sometimes we may wish to find the centroid of a figure or solid consisting of component parts with known centroids.